I recently came back from a trip to the south island (in January), and was there a few years back in March, and have a couple of comments:
(1) Milford Sound is lovely, but, in my opinion, the mountains along the road leading there were even more impressive (impossibly high and steep rock cliffs with cascades pouring down them, etc.). Leave plenty of time to spend along the road and the trails leading off from it. (The town of Te Anau is really the only reasonable place to stay the night to explore this region if you don't camp; Queenstown is a loooong slow drive away, though it doesn't look so far on a map.) The early parts of the Routeburn Track were particularly stunning, as were views from the road between there and the Homer Tunnel.
(2) Another place to consider is Paparoa National Park, on the northwest coast. It was only developed fairly recently, so it isn't on too many itineraries yet, but has lots of interesting scenery (strange coastal rock formations, caves and gorges, wonderful jungle rivers, etc.) that hasn't been "done to death" by photographers yet.
(3) If you happen to be taking the road over Arthur's Pass to get to the northwest coast, stop for a few minutes in the Castle Hill area to wander around the weirdly eroded giant boulders that are strewn across the hillsides there. Weirdly photogenic.
General tip: Don't rely just on the comments of others. Find a bunch of photos of the south island (buy a book, search the web, or whatever), and see what appeals to you. Opinions vary. I know that in my case, I found Queenstown, Wanaka and Arthur's Pass to be somewhat disappointing, while other less acclaimed places were pleasant surprises.
Lisa